Hall calls have been assigned to a group of elevator cars by a large number of different strategies. A common strategy estimates the time of arrival (ETA) of each elevator car for a specific hall call to be assigned. A count is computed for each car which represents the time estimated for the car in question to reach the call floor with the proper service direction to serve the hall call. The hall call assignment is given to the car having the lowest ETA count.
The concept of car distribution in an elevator system is apparent when the constantly changing patterns of elevator traffic are considered. If the dispatching strategy keeps the elevators well distributed throughout the building, for conditions other than morning up-peak, they are in a better position to respond to future hall calls. Some strategies park or "spot" cars throughout the building when the traffic has subsided. Spotting, however, is inefficient, as the elevators are doing no useful work as they travel to parking floors, wasting energy and causing unnecessary wear and tear on the cars which increases maintenance costs.
While ETA is a relatively simple strategy calculating ETA times and allowing switching of assignments between cars based upon relative ETA times can lead to poor car distribution. Cars can bunch and race one another to answer hall calls. This leads to leap frogging, and "no-call" stops in which a car stops only to find another car has just arrived to serve the same call. This degrades elevator service, and is wasteful of energy. Thus, it would be desirable to incorporate a method in the ETA algorithm which solves the distribution problem as part of the assignment algorithm itself.